Food allergy versus food intolerance: how to tell the difference
They cause overlapping symptoms but are completely different things — one involves the immune system and can be life-threatening, the other does not. Here is how to tell them apart and why it matters.
People often use "allergy" and "intolerance" interchangeably, but medically they are very different — and confusing them can be dangerous. A true food allergy involves your immune system and can, in the most serious cases, kill within minutes. A food intolerance is unpleasant and can badly affect quality of life, but it is not life-threatening and does not involve the same immune response.
What a food allergy actually is
In a food allergy, your immune system wrongly identifies a harmless food protein as a threat and mounts a defence against it, usually involving an antibody called immunoglobulin E (IgE). The reaction is typically fast — often within minutes and almost always within two hours — and can involve the skin, gut, airways and circulation at the same time.
- Itching, hives (urticaria) or a raised, red rash
- Swelling of the lips, face, tongue or throat (angioedema)
- Wheezing, coughing, chest tightness or difficulty breathing
- Vomiting, stomach cramps or sudden diarrhoea
- In severe cases, a drop in blood pressure, collapse and loss of consciousness
Even a tiny trace of the food can trigger a reaction in someone with a genuine allergy, which is why food labelling law treats the 14 major allergens so seriously.
Anaphylaxis: the emergency everyone should recognise
The most severe allergic reaction is anaphylaxis, and it is a medical emergency. Warning signs include noisy or difficult breathing, swelling of the tongue or throat, a hoarse voice, feeling faint or dizzy, and — particularly in young children — becoming pale, floppy and unusually quiet.
What a food intolerance is
A food intolerance does not involve the immune system. It usually comes down to the body struggling to process a particular food — for example, a shortage of the enzyme lactase makes it hard to digest the lactose in milk. Reactions are generally slower (hours rather than minutes), are dose-related (a small amount may be fine while a large amount causes trouble), and are largely confined to the digestive system.
- Bloating, wind and abdominal pain
- Diarrhoea or loose stools
- Nausea
- Tiredness or feeling generally unwell
Because the symptoms build up gradually and depend on how much you eat, intolerances can be harder to pin down — but they will not cause hives, swelling or breathing difficulty, and they are not life-threatening.
The key differences at a glance
- System involved: allergy = immune system; intolerance = digestive system (usually).
- Speed: allergy = minutes; intolerance = hours.
- Dose: allergy = even a trace can trigger it; intolerance = usually depends on the amount.
- Severity: allergy = can be fatal; intolerance = uncomfortable but not dangerous.
- Diagnosis: allergy = skin-prick or specific-IgE blood tests plus supervised food challenges; intolerance = often an elimination-and-reintroduction diet supervised by a dietitian.
A warning about home testing
Commercial "food intolerance" tests that measure IgG antibodies, or use hair analysis or applied kinesiology, are not supported by evidence and are not recommended by the NHS or by allergy specialists. IgG is a normal response to foods you eat regularly, not a marker of intolerance. These tests frequently produce false positives that lead people to cut out foods needlessly, risking an unbalanced diet.
When to see a doctor
See a GP if you think you or your child has a food allergy — especially if there has ever been any swelling or breathing difficulty, which needs urgent assessment and possibly an adrenaline auto-injector and allergy-clinic referral. For suspected intolerance, a GP or registered dietitian can guide a structured elimination diet safely. Do not cut out major food groups long-term without professional advice, particularly for children, as it can lead to nutritional gaps.